In air systems in which a pneumatically driven device such as an air cylinder is driven by supply air from a compressed air source, when the supply air from the compressed air source is supplied toward the pneumatically driven device through a switching valve or when being discharged from the pneumatically driven device, the air is adiabatically expanded inside piping or the pneumatically driven device, and the temperature is lowered and moisture in the air is condensed to form a mist in many cases. This mist gradually grows and is condensed while supply and discharge of the air is repeated, which causes rusts in the pneumatically driven device or deteriorates a lubricant and disturbs smooth operation of the pneumatically driven device.
In order to prevent the condensation, such an art has been widely known that a dehumidifier device made of a large number of fiber polymer permeation membranes having selective moisture permeability called as hollow fiber membrane in general is used to dehumidify the supply air supplied to the pneumatically driven device and to supply dry air to the pneumatically driven device. The dehumidifier device usually accommodates a hollow fiber membrane module formed by arranging a large number of hollow fiber membranes side by side inside a housing so as to flow purge air for discharging moisture separated around the hollow fiber membrane.
Installation of such dehumidifier devices is an effective measure to prevent condensation even when the temperature is lowered by the adiabatic expansion. But since the dehumidifier device should be installed in a pneumatic line between the compressed air source and the pneumatically driven device, not only that a cost is required for installation of the dehumidifier device but an installation space for the dehumidifier device should be also considered, which further raises the cost.
On the other hand, a semiconductor plant or the like has a pneumatic line through which a low dew-point air (super dry air) is supplied all the time, there are an increasing number of cases that the air is also flown through a pneumatic line to which a pneumatically driven device is connected for the above condensation prevention measure.
However, including installation of the dehumidifier devices, if the fully dehumidified low dew-point air is supplied to the pneumatically driven device, grease used at a portion in the pneumatically driven device where parts are in sliding contact with each other or rotating tends to be easily evaporated and shortens the life of the pneumatically driven device, which is a problem.